1. Field of the Invention
The present invention relates to an image display device which emits light by a luminance control depending on electric potential difference between a first terminal and a second terminal of a driver.
2. Description of the Related Art
Recent years, a display device which employs an organic electroluminescent (EL) element draws attention instead of a liquid crystal display device in the field of the display such as image display. The display device which employs an organic EL element does not need a backlight since it is a self-luminous device, and has functions which surpass those of a liquid crystal display device, for example, high-responsivity, high-contrast, and high-visibility. The display device which employs an organic EL element is also considered to be advantageous in manufacturing cost since the device has a relatively simple structure (see Japanese patent Application Laid-open H08-234683).
For a high-luminance display realization, a large value of electric current needs to be flowing through an organic EL element which constitutes each pixel, since the organic EL element has a light-emitting mechanism depending on an electric current flowing therethrough. However, for avoiding damages to the elements such as organic EL element and thin-film transistors controlling the operation of an organic EL element, the large electric current flowing through the elements is not preferred. An image display device that series-connected plural organic EL elements are arranged for each pixel and improvement of luminance is achieved by emitting light from the plural organic EL elements at once, to avoid the operation with such a large electric current, is proposed.
FIG. 10 is a circuit diagram of a pixel circuit having series-connected plural organic EL elements and included in a conventional image display device. As shown in FIG. 10, the pixel circuit according to the conventional device has an emission mechanism 102 having series-connected organic EL elements 101a to 101d, a first thin-film transistor 103 whose source electrode is connected to the anode side of the emission mechanism 102 and which is a transistor for controlling the electric current value flowing through the emission mechanism 102, a constant potential line 104 connected to the cathode side of the emission mechanism 102, a power supply line 105 connected to the drain electrode of the first thin-film transistor 103, and a capacitor 106 connected between the first thin-film transistor 103 and the power supply line 105. The pixel circuit according to the conventional device has a data line driver circuit 108 supplying the gate electrode of the first thin transistor 103 with a data voltage corresponding to the luminance of the emission mechanism 102 via a data line 107, a second thin-film transistor 109 controlling the electric continuity between the data line 107 and the first thin-film transistor 103 as a switching device, and a scan line 110 connected to the gate electrode of the second thin-film transistor 109 and controlling the operation of the second thin-film transistor 109.
The pixel circuit according the conventional device will be explained. The first thin-film transistor 103 is turned ON and then the voltage corresponding to the display luminance is supplied to the gate electrode of the first thin-film transistor 103 from the data line 107. Since the first thin-film transistor 103 has a function of allowing the source-to-drain current to flow based on the gate voltage supplied, the emission mechanism 102 is supplied with the current according to the voltage supplied via data line 107, and then the emission mechanism 102 emits light with the luminance depending on the supplied current.
However, a problem of the conventional image display device having the series-connected organic EL elements 101a to 101d is that the load of the voltage supply circuit increases according to the increased range of the voltage level supplied via the data line 107. The details on the problem will be described below.
As shown in the circuit diagram of FIG. 10, the organic EL elements 101a to 101d can be considered electrically equivalent to light-emitting diodes, and can be treated as having the same voltage-current characteristics as those of the light-emitting diodes. Therefore, the anode-to-cathode voltage of each of the organic EL elements 101a to 101d has a tendency to increase according to the electric current value, in other words, according to the light emitting luminance. The anode-to-cathode voltage of the whole emission mechanism 102 is also increased according to the increase of the display luminance. As described above, the first thin-film transistor 103 controls the electric current value flowing through the emission mechanism 102 according to its gate-to-source voltage level, and the gate potential is supplied by the data line driver circuit 108 to enable the control.
From the evidences described above, the gate potential Vg supplied from the data line driver circuit 108 can be represented with the potential V0 of the constant potential line 104, the anode-to-cathode potential difference nVOLED (n is the number of organic EL elements) of the emission mechanism 102, and the gate-to-source voltage Vgs of the first thin-film transistor as:Vg=V0+nVOLED+Vgs  (1)where V0 is almost a constant value, and nVOLED and Vgs are various values depending on the display luminance of the emission mechanism 102. More specifically, the gate-to-source voltage Vgs of the first thin-film transistor generally changes within a range from approximately 0 V to approximately 10 V, although the range depends on the structure of the transistor and the usage of the image display device. On the other hand, the variable range of the anode-to-cathode voltage nVOLED of the emission mechanism 102 changes, for example, from approximately 20 V to approximately 70 V when n is approximately 10.
In the conventional image display device, although the band of changes of the gate-to-source voltage of the thin-film transistor 103 which is necessary for the emission of the emission mechanism 102 at predetermined luminance is approximately 10 V, the data line driver circuit 108 must operate the potential supply within a large range from approximately 20 V to approximately 80 V because of the effect of the potential change at the anode side of the emission mechanism 102, therefore, the load of the data line driver circuit 108 is increased.